Conventionally, based on the Carlson method a color image is formed in a plurality of cycles. According to the Carlson method, a color image is formed as disclosed in Japanese Patent Publication Open to Public Inspection No. 27560/1986 (referred to as Official Gazette 1 in this specification), which will be described below. The photoreceptor drum is composed of a drum made of aluminum, on which a photoconductive layer is provided. Around the photoreceptor drum, there are provided a single charger, a single exposing unit and a plurality of developing units. When the photoreceptor drum is rotated by a plurality of times, a plurality of color toner images are superimposed on the photoconductive layer. Then the superimposed color image is transferred onto a transfer sheet by one operation. Next, the transferred image is fixed onto the transfer sheet. In this way, the color toner image is formed.
According to the image forming apparatus described in Official Gazette 1, the toner image of each color can be superimposed with high accuracy, so that the occurrence of color doubling can be advantageously avoided. However, it is necessary to rotate the photoreceptor drum by a plurality of times for image formation. Accordingly, the image forming speed is low and the working efficiency can not be enhanced.
When a plurality of toner images of different colors are superimposed on the photoreceptor drum, image exposure is blocked by a previously formed color image. Therefore, the image color reproduction is deteriorated.
Further, according to the image forming apparatus described in Official Gazette 1, the circumferential length of the photoreceptor drum must be longer than the length of the transfer sheet. For example, when a transfer sheet of size A3 is used, the diameter of the photoreceptor drum must be 180 to 200 mm. Therefore, dimensions of the apparatus are increased.
For example in Japanese Patent Publication Open to Public Inspection No. 307307/1993 (referred to as Official Gazette 2 in this specification), the following technique is disclosed. The photoreceptor drum is composed of a transparent cylindrical support on which a photoconductive layer is provided through a transparent conductive layer. Outside the photoreceptor drum, there are provided a plurality of chargers and developing units. Inside the photoreceptor drum, there are provided a plurality of LED exposure units. A color image is formed on the photoreceptor drumby one cycle operation.
According to Official Gazette 2 described above, the diameter of the photoreceptor drum can be made 60 to 160 mm. Therefore, the dimensions of the image forming apparatus can be reduced and also the weight thereof can be reduced. Further, the image forming process can be simplified. As a result, it becomes possible to form a color image at high speed.
However, recently, in the field of electrophotography, there is an increasing demand for a small-sized, light weight and highly durable image forming apparatus capable of forming an image of high quality. In the case of a color image forming apparatus, the color reproduction must be high, and further it is required that the color balance is excellent.
According to the method disclosed in Official Gazette 2, color image formation can be accomplished by one-cycle operation. Therefore, processes of charging, exposing and developing are continuously conducted in a short period of time. For this reason, the dynamic sensitivity of the photoreceptor must be high, and further it is important that the charging rises quickly.
When the charging onto the photoreceptor surface dose not rise quickly, even in a charging process in which a sufficiently high charging potential ought to be obtained, the predetermined surface potential can not be provided by the start of image exposure, and even in the process of image exposure, the electric potential of the photoreceptor gradually rises. As a result, it is difficult to form a clear image of high density. When the dynamic sensitivity of the photoreceptor is low, an electrostatic latent image formation can not be completed by the start of development after exposure. Accordingly, in the image formation process, the residual potential and image are increased. As a result, the image quality and color reproduction are low. Therefore, it is impossible to form a clear color image of high quality.
As described above, when the color image forming apparatus described in Official Gazette 2 is put into practical use, various problems may be encountered. Therefore, it is very important to improve the characteristic of the photoreceptor to be used.
However, in Official Gazette 2, there are no descriptions of the photoreceptor suitable for the color image formation conducted in one-cycle operation.